A spark plug for internal-combustion engines is mounted on an internal-combustion engine, and is used to ignite an air-fuel mixture in a combustion chamber. Generally, a spark plug is provided with an insulator having therein an axial bore, a center electrode inserted in the axial bore, a metal shell formed in an outer circumference of the insulator and a ground electrode provided on a front end face of the metal shell to form a spark discharge gap with the center electrode.
Further, it has been disclosed that a noble metal tip made of a noble metal alloy, such as platinum, is joined to a front end portion of the ground electrode made of a heat and corrosion resistant metal, such as a nickel alloy, so as to improve durability of a spark plug
See, for example, Japanese Patent Application Laid-Open (kokai) No. 2003-323962.
However, crystal grains of platinum tend to get rough and large (grain growth) at a high temperature. When the grains grow, grain boundary intensity deteriorates. Thus, vibration accompanying an engine operation or heat cycles in the engine is likely to cause cracking of a noble metal tip, resulting in a separation of the noble metal tip.
In recent years, although the noble metal tip is formed so as to project from the ground electrode in order to improve ignitability and flame propagation property thereof, this configuration tends to cause deterioration in heat conduction of the noble metal tip, resulting in the noble metal tip having a high temperature. Therefore, the grain growth is more likely to advance, and the separation of the noble metal tip is more likely to occur.
The present invention has been accomplished in view of the above-mentioned problems. An advantage of the present invention is a spark plug for internal-combustion engines that is capable of preventing a separation of a noble metal tip and extending a service-life of the spark plug.